Secondary zinc sources are an economic source of zinc compared to primary zinc. Such secondary sources include steel plant dusts, Zinc dross, zinc residues and other sources of secondary zinc. It would be desirable to use zinc-containing steel plant dusts in that the disposal of steel plant dusts is quite expensive. Such dusts have been classified by the EPA as hazardous because of the cadmium and lead content and, therefore, require special disposal sites.
A very common use of zinc is in providing a protective coating to steel. Close to 50% of the annual consumption of zinc is in the production of zinc-coated steel products. Zinc provides excellent resistance to corrosion in all types of air atmospheres, which property is one of the most broadly exploited. Traditionally, the most common process is hot dip galvanizing.
However in recent years, electrogalvanizing has become more popular. In this process, zinc is electrolytically deposited from an aqueous solution on the metal part to be coated.
Electrogalvanizing offers several advantages. Some of these are:
1. The electrodeposited zinc surface has a smoother, more uniform finish and is more suitable for further deposition of alloys such as zinc-nickel or zinc-iron alloys. The electrodeposited coating also provides a surface that has improved paintability compared to hot dip galvanized steel.
2. Savings can be realized by reducing the thickness of the coating and by plating only one side of the strip or sheet. Such sheets and strips find wide applications in automobiles and household appliances.
3. There are no adverse metallurgical effects on the base material, as is the case with thermal effects due to using molten metals.
4. The technique is particularly suited to the protection of small components, pressings and threaded fasteners which cannot be hot dip galvanized due to distortion or clogging of threads.
In the past, it has not been uncommon to use zinc cyanide baths for plating. However, with increasingly stringent environmental regulations, the emphasis is shifting towards zinc sulfate baths. Two options are available to supply the zinc to the bath. In the first option, zinc is supplied as a consumable anode. In the second option, a non-consumable anode such as coated titanium or lead is used as the anode. Zinc is added to the electrolyte in the form of pure zinc by leaching pure zinc metal, oxide or carbonate in spent electrolyte.
The technology of using insoluble electrodes is well known. Reference is made to a paper entitled "High-Speed Electrogalvanizing Line With Insoluble Anodes At Kimitsu Works Of Nippon Steel Corporation" by M. Morimoto et al., Proceedings Of 2nd Continuous Plating Seminar, American Electroplating Society, Winter Park, Fla., 1977, pp. 5-8. Carbon, lead alloy, metals of the platinum group and the like are mentioned as usable materials as insoluble anodes. This paper is incorporated herein by reference.